A 26-year-old man reported with a tender, 12 × 10 mm elastic, hard, movable nodule covered by a rough surface on his left cheek [[Figure 1]a and b]. Biopsy disclosed a poorly circumscribed dermal lesion comprised of keratin-filled cystic structures lined by two layers of epithelial cells and small epithelial cords surrounded by mucoid, hyalinized, and desmoplastic stroma. It extended into the subcutis [Figure 2]. The diagnosis was Microcystic adnexal carcinoma (MAC). Cervical lymph nodes were not palpable and CT showed no metastatic lesions. MRI confirmed that invasion was limited to subcutaneous tissue. The tumor was removed surgically with a 10-mm lateral margin; we also removed part of the underlying facial muscle layer. The lateral and vertical margins were free from the lesion; there was no recurrence at nine-month follow-up. Besides findings typical of MAC, histological examination of the tumor identified immature hair follicles, and mature sebaceous- and ectopic apocrine glands in the dermis, findings suggestive of a pre-existing Nevus Sebaceous (NS) [Figure 3]. A photograph of the patient at age 13 revealed a freckle at the site of the nodule [Figure 1]c, suggesting that his MAC developed at the site of the NS.

Figure 1: (a and b) The patient at 26 years of age. (c) The patient at age 13, there was a freckle on his left cheek

Our review of the literature found only two earlier cases with MAC arising within a NS. [1],[2] MAC was described as a slow-growing uncommon adnexal malignant neoplasm with follicular and sweat duct differentiation and as a sclerosing variant of ductal eccrine carcinoma. [3] Histologically, its two components within a desmoplastic stroma, basaloid keratinocytes containing horn cysts and abortive hair follicles and duct- or gland-like structures, [3] are arranged in cords, are island-shaped, or exhibit a duct-like configuration.

Jadassohn described NS as a hamartomatous locus of an embryologically defective pilosebaceous unit. [4] They are often present at birth. In adolescence, mature sebaceous glands and papillomatous hyperplasia of the overlying epidermis become evident and ectopic apocrine glands develop in about two thirds of individuals. At any stage of life, typically immature hair follicles with incompletely differentiated hair structures are seen. Various types of appendage tumors can develop secondarily within the NS lesions.

The pathological relationship between MAC and NS remains unclear. During embryonic epidermal differentiation, primitive embryonic cells separate into basal cells, hair germs, and eccrine primordium units. The hair germ develops into folliculo-sebaceous-apocrine units and subsequently forms the epithelia of hair follicles, sebaceous- and apocrine glands, and ducts. The eccrine primordium unit differentiates into eccrine glands and ducts. NS is a compound epithelial nevus involving the epidermis, pilosebaceous units, and apocrine glands. Primary epithelial germ cell defects are the basis for the abnormalities of NS and their tendency for neoplastic growth.

MAC is classified as a malignant neoplasm with pluripotential adnexal keratinocytes. [3] Others [5] reported that MAC shows eccrine differentiation and regarded it as a well-differentiated syringomatous carcinoma. While the origin of MAC remains unclear, some adnexal tumors manifest concomitant pilosebaceous-, apocrine-, and eccrine differentiation. Considering the embryogenesis of the epidermis and appendages, we posit that adnexal tumors develop due to disorganized or abnormal differentiation of pluripotent cutaneous stem cells and that MAC arises phylogenetically from NS.